Field of the Invention
One disclosed aspect of the embodiment relates to image forming apparatuses, and, more particularly, to an image forming apparatus in which an alternating-current voltage is applied to a charging member.
Description of the Related Art
In image forming apparatuses that use an electrophotographic method or an electrostatic recording method, a corona charger has been used as a charging unit for charging an image bearing member such as an electrophotographic photosensitive member or an electrostatic recording dielectric member.
In recent years, contact charging devices that employ a method of charging an image bearing member, which is a member to be charged, by bringing a charging member to which a voltage is applied in contact with the image bearing member have been put to practical use for the purpose of the reductions in ozone emission and power consumption.
Examples of the charging method employed in such a contact charging device include a “DC charging method” of charging a member to be charged via a charging member to which only a DC voltage is applied and an “AC charging method” of charging a member to be charged via a charging member to which a voltage including an AC voltage component and a DC voltage component and whose voltage value periodically changes with time is applied. In current years, the “AC charging method” that is excellent in charging uniformly is widely used.
In an image forming apparatus in which charging control is performed using the AC charging method, a positive voltage and a negative voltage are alternately applied and discharge and reverse discharge are repeated. Because of the discharge, the surface of a photosensitive drum, which is a member to be charged, is deteriorated. The deteriorated surface of the photosensitive drum is worn by friction with a contact member such as a cleaning blade. For example, Japanese Patent Laid-Open No. 2014-106459 discloses an apparatus for properly controlling the amount of discharge current in the AC charging method.
In an image forming apparatus disclosed in Japanese Patent Laid-Open No. 2014-106459, a discharge current is detected by a plurality of band-pass filters and a discharge current component is directly controlled as illustrated in FIG. 2.
On the other hand, in an image forming apparatus, in a case where a print speed is set to the maximum print speed (normal speed mode), a half speed (half speed mode), or a one-third speed (one-third speed mode) in accordance with various conditions, an AC frequency applied at the time of charging may be changed. In a case where the technique disclosed in Japanese Patent Laid-Open No. 2014-106459 is applied to this situation where there are a plurality of charging voltages corresponding to the AC frequencies, it can be considered that band-pass filters corresponding to the charging voltages are provided. However, this leads to the increase in cost.